A system for the wireless transmission of multiple information signals utilizing digital time division circuits between a base station and a plurality of subscriber stations. The subscriber stations may be fixed or movable. The movable subscriber stations may be relatively fast and relatively slow moving. The number of time division circuits is determined by the transmission quality of the signals. The base station is interconnected with an external information network, which may be analog and/or digital. The information signals are selected from the group consisting of voice, data, facsimile, video, computer and instrumentation signals. The modulation level of the signals and the power applied to the system are adjusted in accordance with signal error detection in the system. The system is provided with spatial diversity by using a plurality of antennas selectively spaced from each other to provide relatively high signal reception despite signal fading. The base station operates over a plurality of RF channel pairs. Each channel pair operation is implemented by the combination of a transmit channel circuit for processing a given plural number of information signals received simultaneously over telephone company trunk lines for simultaneous transmission to different subscriber stations over a given radio frequency (RF) channel, and a receive channel circuit for processing a plurality of signals received simultaneously over a given RF channel from different subscriber stations to provide information signals for transmission over the trunk lines.
Separate conversion devices are respectively connected to each of the trunk lines for converting the information signals received over the trunk lines into digital signal samples.
The transmit channel circuit includes a given plural number of separate signal compression devices for simultaneously compressing the digital signal samples respectively derived from separate ones of the conversion devices to provide the given number of separate compressed signals; a channel control unit connected to the compression devices for sequentially combining the compressed signals into a single transmit channel bit stream, with each of the respective compressed signals occupying a repetitive sequential slot position in the transmit channel bit stream associated with a predetermined one of the separate compression devices and a unit for providing a transmit channel signal for transmission over the predetermined RF channel in response to the transmit channel bit stream.
An exchange couples the respective separate conversion devices to indicated ones of the separate compression devices.
A remote-connection processor unit is coupled to the trunk lines and responds to an incoming call request signal received over one of the trunk lines by providing a slot assignment signal indicating which one of the separate compression devices the exchange is to connect to the one of the separate conversion devices connected to the one trunk line, and thereby assigns to the one trunk line the slot in the transmit channel bit stream associated with the one of the separate compression devices that is so connected by the exchange. The remote-connection processor maintains a memory of which slots are so assigned and consults such memory upon receipt of an incoming call request and then provides the slot assignment signal that effects the connection to a compression device associated with one of the slots that is not assigned to another trunk line.
A call processor is connected to the remote-connection processor and responds to the slot assignment signal by causing the exchange to complete the connection indicated by the slot assignment signal.
The receive channel circuit includes a receiver unit for receiving a receive channel signal and for processing the receive channel signal to provide a receive channel bit stream containing separate compressed signals in different respective repetitive sequential slot positions; a given plural number of separate signal synthesis devices each associated with a different slot position in the receive channel bit stream for reconstructing digital signal samples from separate compressed signals contained in the associated respective slot positions of the receive channel bit stream; and a control unit for segregating the separate compressed signals from the receive channel bit stream and distributing the segregated signals to the separate synthesis devices associated with the respective time slots from which the signals are segregated.
Separate reconversion devices are respectively connected to each of the trunk lines for reconverting digital signal samples to information signals for transmission over the respective trunk lines. Each oft he separate reconversion means is associated with one of the separate conversion means and is connected to a common one of the trunk lines with the associated separate conversion means.
The exchange couples the respective separate reconversion devices to indicated ones of the separate synthesis devices.
The remote-connection processor responds to the incoming call request signal received over the one trunk line by providing a slot assignment signal for indicating which one of the separate synthesis devices the exchange is to connect to the one of the separate reconversion devices connected to the one trunk line and thereby assigns to the one trunk line the slot in the receive channel bit stream associated with the one of the separate synthesis devices that is so connected by the exchange means. The remote-connection processor maintains a memory of which slots in the receive channel bit stream are so assigned and consults such memory upon receipt of the incoming call request and then provides to the call processor the slot assignment signal for effecting the connection to a synthesis device associated with one of the slots that is not assigned to another trunk line.
The system of the present invention makes use of advanced digital and large-scale-integration electronics techniques to bring low-cost, reliable, high-quality communications to various market segments. One preferred embodiment uses a fixed base station installation centrally located to communicate with a large number of subscriber stations placed in the nearby geographic area. The central base station may be connected to a central office of a public switched telephone company (Telco) network through a private branch exchange switch (PBX) connected to incoming telephone trunks. The subscriber stations in the system may be either fixed-base portable or mobile in nature, and may be operational in either relatively slow or fast motion. The subscriber stations communicate with the base station via UHF radio channels and with the user via standard two-wire DTMF touchtone telephone equipment or via RS-232C or via non-standard phone stations (e.g., 4-wire). The system may be used to replace existing hard-wired local subscriber loops or to provide quality telephone service to areas where wired connections are not feasible or economical.
One feature of the system of the present invention is the ability to use time division multiple access (TDMA) and digital speech encoding to allow simultaneous multiple use of frequencies within a given network. Any feasible number of high-quality voice circuits may be operating on a given frequency channel (with 25 KHz channel spacing) at a time. Four such circuits are used for illustrative purposes. This provides both a spectral and an economic advantage over existing analog radio-phone systems which can provide for only one conversation at a time on a given frequency channel.
Features which bring about the lower-cost fixed, mobile and portable service are the use of low-rate digital voice coding (less than 16 Kbps) combined with spectral-efficient digital modulation techniques. For example, the combined use of a 14.6 Kbps voice coding technique and 16-level DPSK modulation allows four simultaneous full-duplex conversations to be supported on a single pair of 20 KHz Bw channels that are spaced 25 KHz apart in the entire spectrum, and particularly in the 400-500 MHz and 800-950 MHz segments. This combination provides good quality speech over a distance of at least 20 Km.
To be competitive with wireline service, a much larger population of subscribers must be accommodated than can be supported simultaneously on a given pair of 25 KHz channels. For example, a 12-channel-pair system which supported 47 simultaneous calls might have a total off-hook plus on-hook population of 500 subscribers (with the maximum constrained by the desired peak-hour blocking probability). Thus, a subscriber call-request control scheme which provides reasonable call-connection delays is also an important feature of the present invention.
Additional features of the present invention are described in relation to the description of the preferred embodiments.